I was in Kona for the first time last month. It was a short trip, with only two dive days. I dove with Big Island Divers on both mornings, and dove with Kona Honu Divers for the Manta dive (Unfortunately, no mantas showed up, but we still got in a really nice night dive). Both operations did a great job, in my opinion. Personally, I liked Big Island Divers better, but that's because I usually prefer a smaller boat.
My Hawaiian diving experience is limited to two idlands -- Maui (lots of dives) and the Big Island/Kona side (only 6 dives). Based on this limited experience, plus what I've heard from the grape vine and read here on Scuba Board, most dive ops in the Aloha State do a great job. Folks will have their own favourite ops, but it has to do with an almost intangible feeling of being more "at home" with a specific operation's style.
In any case, please let us know about your Kona trip when you get back. I'm hoping to get back to Kona in the not to distant future, and would love to hear about your experiences there, as well as your recommendations.
Peace,
Dave
PS regarding the Manta dive -- Based on my very limited experience, it might be good to ask about manta numbers over the previous 2 or 3 nights. When I went, they told us that they'd had 6 mantas two nights earlier and 3 mantas on the previous night. I figured that the good old "Bittner Luck" (my family's name for Murphy's Law when one of us is involved) would kick in, and sure enough, no mantas showed. However, since I (sadly) won't be on the Big Island over Christmas, you'll probably see wall-to-wall mantas!